The invention relates generally to x-ray tubes, and more particularly to structures and methods of assembly for the bearing utilized in an x-ray tube.
X-ray systems may include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, may be located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. The object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes include a cathode and an anode located within a high-vacuum environment. In many configurations, the anode structure is supported by a liquid metal bearing structure, e.g., a spiral groove bearing (SGB) structure, formed with a support shaft disposed within a sleeve or shell to which the anode is attached and that rotates around the support shaft. The spiral groove bearing structure also includes spiral or helical grooves on various surfaces of the sleeve or shell that serve to take up the radial and axial forces acting on the sleeve as it rotates around the support shaft.
Typically, an induction motor is employed to rotate the anode, the induction motor having a cylindrical rotor built into an axle formed at least partially of the sleeve that supports the anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. The x-ray tube cathode provides a focused electron beam that is accelerated across an anode-to-cathode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearings and the material forming the anode structure, i.e., the anode target and the shaft supporting the target.
Advantages of liquid metal bearings such as spiral groove bearings in x-ray tubes include a high load capability and a high heat transfer capability due to an increased amount of contact area. Other advantages include low acoustic noise operation as is commonly understood in the art. Gallium, indium, or tin alloys, among others, are typically used as the liquid metal in the bearing structure, as they tend to be liquid at room temperature and have adequately low vapor pressure, at operating temperatures, to meet the rigorous high vacuum requirements of an x-ray tube. However, liquid metals tend to be highly reactive and corrosive. Thus, a base metal that is resistant to such corrosion is desirable for the components that come into contact with the liquid metal bearing, such as the shaft of the anode assembly and is rotated for the purpose of distributing the heat generated at a focal spot.
As a result, the structure of the sleeve to which the anode is connected and the support shaft must be capable of withstanding the high temperatures and mechanical stresses created within the x-ray tube, as well as be able to withstand the corrosive effects of the liquid metal bearing. As such, a refractory metal such as molybdenum or tungsten is typically used as the base material for the construction of the sleeve or shell as well as for the other bearing components. Not only are such materials resistant to corrosion and high temperatures, but they tend to be vacuum-compatible and thus lend themselves to an x-ray tube application. In addition, because liquid metal bearings require geometries and/or tolerances that maintain fluid gaps between bearing surfaces on the order of single micrometers, using a highly thermal conductive and low expanding material such as molybdenum to form the bearing components enables the size of these gaps to be maintained despite high thermal gradients resulting from target heating.
However, as the refractory materials are difficult to machine, these surfaces are hard to manufacture without surface imperfections that enable leaks to occur in the seals. Also, due to the low galling/wear properties of the refractory materials, these surface imperfections, even if not present after machining, can occur during normal use of the tube resulting in the formation of fluid leaks, thereby shortening the useful life of the tube.
In an alternative construction for a liquid metal/spiral groove bearing structure, other metals, such as steel, can be utilized in place of the refractory metals for the construction of the sleeve and support shaft, such as disclosed in U.S. Pat. No. 6,477,236. While these other metals have a lower thermal conductivity, they have the benefits of low cost compared to the refractory metals, good machinability, good galling/wear characteristics, and good weldability. In particular, steel is a potential journal bearing material in x-ray tubes as it has better wear resistance compared to molybdenum. As such, these metals can be more easily constructed and joined to form the bearing sleeve.
However, one drawback to steel is that it has a much lower thermal conductance and higher coefficient of thermal expansion compared to molybdenum, making steel more prone to thermal gradients and resulting non-uniform bearing deflections, which in turn makes maintenance of the fluid gap sizes difficult. Further, another challenge presented by these properties of a steel bearing is that the steel parts expand more during use. This increased thermal deformation of the steel bearing components can directly result in bearing failure, such as though expansion of the gap creating leakage of the liquid metal lubricant. Additionally, the thermal deformation/expansion causes movement in the anode parts of the bearing, particularly in and through the thrust flange of the shaft, resulting in more movement of the focal spot than with a refractory metal bearing structure.
Therefore, it is desirable to develop a structure and method for the formation and operation of a bearing structure for an x-ray tube with an improved cooling structure to enable the use of low cost materials for the shaft to significantly improve heat transfer out of the bearing structure to minimize the thermal gradients and resulting deformation expansion and non-uniform bearing deflections in the bearing assembly structure.